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In this article, I address the epistemological role that geometry plays in Kepler’s Harmonices MundiLibri V and argue that the framework he develops there is meant to address concerns regarding the confirmation of astronomical hypotheses, which are supported by comments in earlier works regarding empirical underdetermination. The geometrical epistemology that he constructs to combat these concerns in the Harmonices Mundiis introduced in Book I and then is extended to his theory of harmonic proportion in Book III, finally providing the foundation for his derivation of the planetary motions in Book V. To argue for these claims, I begin by discussing Kepler’s concern with underdetermination in earlier works. Then I turn to the Harmonices Mundiand argue that Kepler seeks to provide a geometrical system that directly links the theorist with the world. Finally, I show how he applies this system to his astronomy via his harmonic theory. This account not only helps us to understand Kepler’s scientific methodology better but also sheds light on Kepler’s enthusiasm for the results of the Harmonices Mundiby showing how they provide an example of the successful application of his geometrical epistemology.